i always fill up with premium from walmart or other grocer's, then i pour in a 2 dollar bottle of Oreilly's Octane Booster. With that combination, my truck runs great. I put regular in the tank once when i first bought my truck and it got 16 MPG on the Highway, ran it almost dry and filled it up with premium next and gas milage was up to 22mpg. Not sure exactly what caused such a jump, but i'm pretty sure most newer vehicles are optimied for premium anyways